Night vision systems allow a vehicle occupant to better see objects during relatively low visible light level conditions, such as at nighttime. Night vision systems typically are classified as either passive night vision systems or active night vision systems. Passive systems simply detect ambient infrared light emitted from the objects within a particular environment. Active systems utilize a near infrared (NIR) light source to illuminate a target area and subsequently detect the NIR light reflected off objects within that area.
Passive systems typically use far-infrared cameras that are characterized by low resolution and relatively low contrast. Such cameras must be located on the vehicle exterior in order to acquire requisite infrared energy in the operating environment. Externally mounted cameras can negatively affect vehicle styling. Far-infrared cameras are also costly to manufacture and generate non-intuitive images that can be difficult to interpret.
Active systems provide improved resolution and image clarity over passive systems. Active systems utilize laser or incandescent light sources to generate an illumination beam in the near infrared spectral region and charge-coupled devices or CMOS cameras to detect the reflected NIR light.
Diode lasers are preferred over incandescent light sources for several reasons. Incandescent light sources are not monochromatic like diode lasers, but instead emit energy across a large spectrum, which must be filtered to prevent glare onto oncoming vehicles. Filtering a significant portion of the energy generated from a bulb is expensive, energy inefficient, and generates undesired heat. Also, filter positioning is limited in incandescent applications, since the filter must be located proximate an associated light source. As well, multiple incandescent sources are often required to provide requisite illumination, thus increasing complexity and costs.
In an active night vision system a NIR laser is used to illuminate a target area. A camera is used in conjunction with the laser to receive reflected NIR light from objects within the target area. The laser may be pulsed with a duty cycle of approximately 25-30%. The camera may be operated in synchronization with the laser to capture an image while the laser is in an “ON” state.
The camera typically contains a band-pass filter that allows passage of light that is within a narrow range or band, which includes the wavelength of the light generated by the laser. The combination of the duty cycle and the use of the band-pass filter effectively eliminates the blinding effects associated with headlamps of oncoming vehicles. The term “blinding effects” refers to when pixel intensities are high due to the brightness of the oncoming lights, which causes an image to be “flooded out” or have large bright spots such that the image is unclear.
Although laser-based night vision systems can overcome the blinding effects of oncoming headlamps, they cannot overcome blinding effects associated with highly reflective objects. For example, many signs have highly reflective surfaces for reflection of incandescent light, such as emitted from vehicle headlamps, for direct viewing ease by a vehicle operator. The signs are often covered with retroreflective paint that can reflect a large amount of light and cause image saturation. A saturated image is generally unclear and unreadable. Large flat surfaces, such as on trucks, buses, and vans, can also cause image saturation. Image saturation from the stated flat surfaces typically occurs when the flat surfaces are within approximately 20-40 m of the host vehicle.
Thus, there exists a need for an improved active night vision system and method of generating images that minimizes or eliminates the blinding and saturating effects due to the presence of highly reflective objects within a target area of a vehicle.